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The reactivity and toxicity of the soluble toxicants in the presence of the engineered nanomaterials is not well explored. In this study, the probable effects of TiO₂ and Al₂O₃ nanoparticles (n-TiO₂, n-Al₂O₃) on the toxicity of Cr(VI) were assessed with the dominant freshwater algae, Scenedesmus obliquus, in a low range of exposure concentrations (0.05, 0.5 and 1 μg/mL). In the presence of 0.05 μg/mL n-TiO₂, the toxicity of Cr(VI) decreased considerably, which was presumably due to the Cr(VI) adsorption on the nanoparticle surface leading to its aggregation and precipitation. The elevated n-TiO₂ concentrations (0.5 and 1 μg/mL) did not significantly influence Cr(VI) bio-availability, and a dose dependent toxicity of Cr(VI) was observed. On the other hand, n-Al₂O₃ did not have any significant effect on the Cr(VI) toxicity. The microscopic observations presented additional information on the morphological changes of …